The present invention relates generally to adhesive compositions and more particularly to adhesive compositions of the type which undergo a change from colored to colorless upon application to a substrate.
Adhesive compositions of the type which are in a rigid stick-form, which are slidably mounted in a reclosable dispensing tube and which deposit a tacky film when rubbed onto a receiving substrate are well-known. An example of such an adhesive composition is described in U.S. Pat. No. 3,576,776, inventors Muszik et al., which issued Apr. 27, 1971 and which is incorporated herein by reference. The adhesive composition of the aforementioned patent comprises a shape-giving base consisting of a gel of a gel-forming salt of an aliphatic carboxylic acid having from 8 to 36 carbon atoms with a cation selected from the group consisting of alkali metals, ammonia, and lower alkylamines. The adhesive composition also comprises a liquid selected from the group consisting of water, water-miscible organic solvents and mixtures thereof, and further comprises an amount of an adhesive component selected from the group consisting of water-soluble and water-dispersible adhesives. As described in the foregoing patent, a preferred example of the gel-forming salt is sodium stearate, and a preferred example of the water-soluble or water-dispersible adhesive is polyvinylpyrrolidone (PVP).
Adhesive sticks of the type described above suffer from the problem that they are colorless, thus making it difficult for a user to identify where the adhesive has been applied on a substrate. While the incorporation of a color dye or pigment into the adhesive composition would aid in identifying those areas coated, the presence of such colors in the adhesive would markedly limit the use of such adhesives since the color would remain visible, in many instances, if white or light colored substrates are adhered therewith.
One approach to the above-identified problem has been to include a colored indicator in the adhesive composition which, upon being applied to a substrate, undergoes a change in color from colored to colorless. Accordingly, immediately after applying the adhesive composition to a substrate, a user can identify where the adhesive has been applied based on its color; however, shortly thereafter, the adhesive dries colorless. Examples of the indicator include phthaleins, such as phenolphthalein and its alkyl derivatives. An adhesive composition of the type described above is disclosed in U.S. Pat. No. 4,954,544, inventor Chandaria, which issued Sep. 4, 1990 and which is incorporated herein by reference. Another adhesive composition of the type described above was sold by the present assignee, Avery Dennison Corporation, under the trademark AVERY.RTM. Disappearing Color Glue Stic. The AVERY.RTM. Disappearing Color Glue Stic adhesive composition included a base of sodium stearate in water, PVP, phenolphthalein, propylene glycol, polyethylene oxide and a plurality of antimicrobial agents.
One problem that was recognized with the aforementioned AVERY.RTM. adhesive composition was that, after being stored for a period of time, when the adhesive composition was applied to a substrate, it would not dry colorless, but instead, would dry to a brown color. As can readily be appreciated, this result was highly undesirable from a cosmetic point of view. Another problem that was recognized with the abovementioned adhesive composition was that, after being stored for a period of time, the adhesive composition, even before being applied to a substrate, often would lose the fullness of its color and, instead, would turn a more pale color.